Light Emitting Diodes (LEDs) have sparked interest in their use for illumination. Unlike incandescent light sources, which are broadband blackbody radiators, LEDs produce light of relatively narrow spectra, governed by the bandgap of the semiconductor material used to fabricate the device. One way of making a white light source using LEDs combines Red, Green, and Blue (RGB) LEDs to produce mixed (e.g., white) light. Slight differences in the relative amounts of each color of the RGB based light source manifest as a color shift in the light. Use of an RGB based light source to replace existing light sources requires that the color of the light be controlled and constant over the lifetime of the unit.
RGB based light sources are widely used for Liquid Crystal Display (LCD) back-lighting, commercial freezer lighting, white light illumination, and other applications. Some applications require more careful control of spectral content than others and differing color temperatures may be desired for different applications. For careful control of spectral content, feedback control mechanisms are sometimes used to ameliorate differences between LEDs. Such differences may be due to the aging of the LEDs, variations in temperature, or shifts in drive currents. Even LEDs manufactured by nominally identical processes often have slight variations vis-à-vis one another.
Unfortunately, light guide design becomes increasingly complex, and accurate feedback increasing problematic, as display panels increase in size or incorporate multiple light sources. When a light guide is large, as may be the case for sizeable LCD panels or window glass, ensuring adequate color uniformity across a display is a significant challenge. Moreover, for light guides designed to transport light from multiple sources to a feedback point, careful light guide panel design is required to couple the light output from each light source to the feedback point.